OFDM系统中相位噪声的影响及ICI自消除方案

相位噪声也能引起信道间干扰( ICI) ,本文系统的分析了采用ICI自消除方案前后相位噪声对OFDM系统的影响,得出了载波干扰比和信噪比的近似计算公式,在AWGN信道上仿真了系统的误码性能。从仿真结果看,ICI自消除的方案能够显著提高系统的载波干扰比和误码率     

CO-OFDM系统中基于维纳滤波的相位噪声ICI抑制算法

针对相干光正交频分复用(CO-OFDM)通信系统中相位噪声引起的载波间干扰(ICI)问题,提出一种基于维纳滤波的相位噪声ICI抑制算法。算法不需要改变OFDM符号原有的结构,只需要利用相位噪声预先得到的统计数据,通过相关计算,得到ICI的修正项。由于没有在OFDM符号中添加额外的辅助数据,所以与ICI自消除算法相比,所提出的ICI抑制算法具有较高的频谱利用率。仿真结果表明,在相位噪声比率为10-4级别时,基于迭代检测的ICI消除算法的误码率(BER)趋于10-4,出现错误平层;而本文所提出的ICI抑制算法能够有效地降低错误平层,改善BER性能。     

OFDM系统中ICI自消除算法的研究

OFDM系统对相位噪声极其敏感，会引入共同相位误差CPE（Common Phase Error）和载渡间干扰ICI（Intercarrier Interference），严重降低系统性能。对ICI自消除算法进行了研究，分析比较了数据取反（data—conversion）ICI自消除算法和数据共轭（data—conjugate）ICI自消除算法性能，并基于此提出了一种改进的取反相位噪声自消除算法。仿真结果表明，该算法实施起来非常简单，且可以有效的降低相位噪声的影响。     

OFDM系统相位噪声的Huber ML抑制方法

OFDM系统对相位噪声(PHN)非常敏感,由其引起的子载波相位旋转和载波间干扰(ICI)会严重恶化系统性能。该文提出了一种新的OFDM接收机PHN估计和抑制方法,利用判决数据采用Huber ML估计方法对PHN引起的ICI分量进行迭代估计,将估计的ICI系数变换到时域来校正每个样点的相位误差,从而达到抑制PHN的目的。与传统的基于判决的估计方法相比,该方法对判决的错误传播具有鲁棒性,可进一步提高ICI抑制性能。对算法在AWGN和多径信道环境下的仿真结果表明所提方法对PHN引起的ICI有较好的抑制性能。     

基于DCT正交基的相位噪声均衡法

相位噪声是困扰OFDM通信系统的问题之一，它会破坏系统子载波之间的正交性，从而引起载波间干扰（ICI），降低系统性能。为此，提出一种时域的相位噪声均衡算法。仿真结果表明它具有良好的性能。     

OFDM系统相位噪声的迭代估计方法

OFDM系统对相位噪声(PHN)非常敏感,由其引起的子载波相位旋转和载波问干扰(ICI)会严重恶化系统性能.本文提出了一种新的OFDM接收机PHN迭代估计方法,利用优化理论(Majorization)得到最大似然(ML)代价函数的优化函数,通过对优化函数求最大化得到ICI系数的迭代估计,将估计的ICI系数变换到时域得到PHN估计,来校正每个样点的相位误差,从而达到抑制PHN的目的.与传统的基于ML的PHN估计方法相比,该方法具有较低的运算复杂度.文章对算法在多径信道环境下进行了仿真,结果表明所提方法对PHN引起的ICI有较好的估计性能.     

OFDM系统中载波间干扰自消除算法研究

OFDM系统对频率偏移极为敏感,频率偏移将致使各个子载波之间失去正交性,导致子载波干扰(ICI),从而降低系统性能。通过对有效抑制ICI的方法—ICI自消除算法进行研究,分析了ICI自消除算法对OFDM系统的影响。研究结果表明:与标准OFDM系统相比,ICI自消除算法使OFDM系统改善了系统误码率。     

OFDM系统中载波间干扰自消除算法研究

OFDM系统对频率偏移极为敏感,频率偏移将致使各个子载波之间失去正交性,导致子载波干扰（ICI）,从而降低系统性能。通过对有效抑制ICI的方法—ICI自消除算法进行研究,分析了ICI自消除算法对OFDM系统的影响。研究结果表明：与标准OFDM系统相比,ICI自消除算法使OFDM系统改善了系统误码率。     

地面数字电视广播系统相位噪声的抑制

将基于二项式消除编码(PCC)的子载波间干扰(ICI)自消除方法与基于导频的公共相位误差(CPE)纠正以及最小均方误差(MMSE)均衡方法相结合.抑制OFDM系统的相位噪声.仿真结果表明,无论在高斯信道还是多径瑞利衰落信道下,本方法都可以有效抑制OFDM系统的相位噪声,同时实现复杂度低,具有较好的实用性.     

OFDM系统中ICI自消除算法研究

OFDM系统对频率偏移极为敏感,频率偏移将导致子载波之间失去正交性,于是产生子载波干扰（ICI）,从而降低系统性能。通过对有效抑制ICI的方法—ICI自消除算法进行研究,分析了ICI自消除算法对OFDM系统的影响。在软件无线电（GNU Radio）平台上搭建该系统,并在实际环境中运行。研究结果表明：与传统OFDM系统相比,ICI自消除算法使OFDM系统的误码率得到改善。     

Robust ICI Self-cancellation OFDM Receiver with Dynamic Phase and Amplitude Estimations

Orthogonal frequency division multiplexing (OFDM) has been applied in the current wireless local-area networks and digital video broadcasting since it is robust against the frequency-selective channels. However, there is still a crucial intercarrier-interference (ICI) problem due to Doppler effect, local frequency drift and sampling clock offset, associated with OFDM systems. Recently ICI self-cancellation schemes have been proposed to significantly reduce the ICI and empirically they greatly outperform the convolutional coding schemes adopted by the IEEE 802.11 standard. However, all existing ICI self-cancellation receivers are still sensitive to the phase and amplitude ambiguities due to the phase offset, the local oscillator frequency drift and the multipath reflections. Therefore, in this paper, a novel receiver design integrating the ICI self-cancellation with a proposed dynamic phase and amplitude estimation mechanism is introduced, which can well solve the ambiguity problem. The Monte Carlo simulation results show that our phase and amplitude estimators can greatly decrease the error probability for the final symbol detection in the ICI self-cancellation OFDM receivers.     

Joint phase/amplitude estimation and symbol detection for wireless ICI self-cancellation coded OFDM systems

OFDM has been applied in the current wireless local-area networks and digital video broadcasting systems since it has the advantage over the conventional single-carrier modulation schemes when the frequency-selective fadings are present. Nevertheless, intercarrier-interference (ICI) due to Doppler frequency drift, phase offset, local oscillator frequency drift, and multipath fading will be a severe problem in OFDM systems. Previous ICI self-cancellation coding schemes can greatly reduce the ICI, but they are very sensitive to the phase ambiguity, which is due to the composite effect of the phase offset, the multipath fading and the local frequency drift. In this paper, the phase ambiguity and amplitude ambiguity problems in ICI self-cancellation coded OFDM receivers will be formulated and discussed. Then, a novel receiver which combines the current ICI self-cancellation coding techniques with a new expectation-maximization-based joint phase/amplitude estimation and symbol detection scheme is proposed. The outstanding performance of this proposed scheme is shown and compared with other existing methods at different noise levels through OFDM simulations.     

Analysis of ICI cancellation scheme in OFDM systems with phase noise

Phase noise in orthogonal frequency division multiplexing (OFDM) systems destroys the orthogonality of the subcarriers and inter-carrier interference (ICI) is caused. In this paper, the ICI self-cancellation scheme is adopted to combat the ICI caused by phase noise in OFDM systems. Moreover, the error coefficients are defined and the theoretical expressions of carrier to interference ratio (C/I) with and without the ICI self-cancellation scheme are separately derived. From the simulation results, it is verified that the ICI self-cancellation scheme obviously decreases the amount of the ICI caused by phase noise and the improvement of C/I could reach 10 dB when the normalized 3 dB bandwidth of phase noise is 0.4. However, the convolutional coding OFDM (COFDM) system could supply more performance gain at the expense of increasing decoder complexity compared to OFDM system with the ICI self-cancellation scheme in the frequency-selective channel.     

General ICI self-cancellation scheme for OFDM systems

One of the challenges in designing orthogonal frequency-division multiplexing (OFDM) systems is their inherent sensitivity to any frequency shift in the signal. A frequency offset between the local oscillators at the transmitter and receiver causes a single frequency shift in the signal, while a time-varying channel can cause a spread of frequency shifts known as the Doppler spread. Frequency shifts ruin the orthogonality of OFDM subcarriers and cause intercarrier interference (ICI); therefore, quickly diminishing the performance of the system. ICI self-cancellation schemes have been proposed to reduce the sensitivity of OFDM systems to frequency shifts. These schemes use signal processing and frequency domain coding to reduce the amount of ICI generated as a result of frequency shifts, with little additional computational complexity. These methods can be used as an alternative to the fine frequency-offset estimation methods to battle oscillator frequency offset or simply be used as an ICI mitigation technique when the system is operating over time-varying channels. We propose a general ICI self-cancellation scheme that can be implemented through windowing at the transmitter and receiver. We show that the previously proposed self-cancellation schemes are equivalent to special cases of this method. Through theoretical analysis of the signal-to-interference ratio and bit-error rate and the use of Monte Carlo simulations, we demonstrate that the proposed system considerably outperforms the existing systems in the presence of frequency offset or time variations in the channel. We consider both coherent and noncoherent systems.     

Joint ICI Cancellation and PAPR Reduction in OFDM Systems Without Side Information

Intercarrier interference (ICI) self-cancellation, new ICI self-cancellation and conjugate cancellation schemes have been proposed in the literature to mitigate the effect of ICI. In this paper we have performed the mathematical analysis of PAPR performances for ICI self-cancellation, new ICI self-cancellation and ICI conjugate cancellation schemes and it is found that PAPR performance of these schemes are either very close to or poorer than the standard OFDM signal, which necessitates the requirement of PAPR reduction. After realizing the need of PAPR reduction in ICI cancellation schemes, we have proposed a joint scheme to reduce ICI and PAPR simultaneously. In this paper, we have proposed a multipoint partial transmit sequence (PTS) scheme, to improve the PAPR performance of ICI cancellation schemes. The proposed multipoint PTS based PAPR reduction scheme is coupled with ICI cancellation schemes in such a way that CIR performance of these schemes after coupling remains unchanged and no SI is required at the receiver to recover the original data signal. A comparison of CIR and PAPR performances for ICI cancellation schemes with and without PAPR reduction is also presented in this paper. The analytical results of CIR and PAPR performances for conventional ICI cancellation and joint ICI cancellation and PAPR reduction confirm the outperformance of the proposed scheme. We have also evaluated the SER performance of the joint schemes over additive white Gaussian noise and fading channels and presented a comparison with other existing schemes.     

Novel ICI cancellation algorithm in OFDM systems

The Bit Error Rate (BER) caused by Inter Carrier Interference (ICI) increases greatly with the increase of frequency offset in Orthogonal Frequency Division Multiplexing (OFDM) systems.According to a typical OFDM signal model,this letter proves that the coefficient matrix of ICI is a unitary matrix whose inverse matrix is much easier to get,and then proposes a new ICI cancellation method with less computation complexity by sending typical pilot symbols.Compared with two existing ICI cancellation methods,self-cancellation and windowing cancellation,it is shown that the proposed algorithm can cancel ICI better and overcome the limitation of the two traditional methods.     

Pilot-free dynamic phase and amplitude estimations for wireless ICI self-cancellation coded OFDM systems

OFDM has the advantage over the conventional single-carrier modulation schemes in the presence of frequency-selective fadings. Nevertheless, intercarrier-interference (ICI) due to Doppler frequency drift, phase offset, local oscillator frequency drift, and sampling clock offset will be a severe problem in the wireless OFDM systems. Previous ICI self-cancellation coding schemes can greatly reduce the ICI, but they are very sensitive to the phase ambiguity, which is due to the composite effect of the phase offset, the multipath fading and the local oscillator frequency drift. In this paper, a novel receiver which combines the current ICI self-cancellation coding techniques with a new pilot-free joint phase/amplitude estimation and symbol detection scheme is proposed. Based on the energy modulation or the irregular symbol constellation, our new technique does not have any requirement of pilot symbols and it can operate on all kinds of phase error ranges. The proposed scheme is promising in comparison with other existing methods at different noise levels through OFDM simulations.     

An Improved ICI Reduction Method in OFDM Communication System

Orthogonal frequency division multiplexing (OFDM) is a promising technique for the broadband wireless communication system. However, the inter-sub-carrier-interference (ICI) produced by the phase noise of transceiver local oscillator is a serious problem. Bit error rate (BER) performance is degraded because the orthogonal properties between the sub-carriers are broken down. In this paper, ICI self-cancellation of data-conjugate method is studied to reduce ICI effectively. CPE (common phase error), ICI and CIR (carrier to interference power ratio) are derived and discussed by the linear approximation of the phase noise. Then, the system performance of the data-conjugate method is compared with those of the original OFDM and the conventional data-conversion method. As results, it can be shown that CPE becomes zero in the OFDM of the data-conjugate method. Besides, in the OFDM system with phase noise, the data-conjugate method can make remarkable improvement of the BER performance and it is better than the data-conversion method and the original OFDM with or without convolution coding.